<?php
/**
 * CCache class file.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @link http://www.yiiframework.com/
 * @copyright 2008-2013 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

/**
 * CCache is the base class for cache classes with different cache storage implementation.
 *
 * A data item can be stored in cache by calling {@link set} and be retrieved back
 * later by {@link get}. In both operations, a key identifying the data item is required.
 * An expiration time and/or a dependency can also be specified when calling {@link set}.
 * If the data item expires or the dependency changes, calling {@link get} will not
 * return back the data item.
 *
 * Note, by definition, cache does not ensure the existence of a value
 * even if it does not expire. Cache is not meant to be a persistent storage.
 *
 * CCache implements the interface {@link ICache} with the following methods:
 * <ul>
 * <li>{@link get} : retrieve the value with a key (if any) from cache</li>
 * <li>{@link set} : store the value with a key into cache</li>
 * <li>{@link add} : store the value only if cache does not have this key</li>
 * <li>{@link delete} : delete the value with the specified key from cache</li>
 * <li>{@link flush} : delete all values from cache</li>
 * </ul>
 *
 * Child classes must implement the following methods:
 * <ul>
 * <li>{@link getValue}</li>
 * <li>{@link setValue}</li>
 * <li>{@link addValue}</li>
 * <li>{@link deleteValue}</li>
 * <li>{@link getValues} (optional)</li>
 * <li>{@link flushValues} (optional)</li>
 * <li>{@link serializer} (optional)</li>
 * </ul>
 *
 * CCache also implements ArrayAccess so that it can be used like an array.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @package system.caching
 * @since 1.0
 */
abstract class CCache extends CApplicationComponent implements ICache, ArrayAccess
{
    /**
     * @var string a string prefixed to every cache key so that it is unique. Defaults to null which means
     * to use the {@link CApplication::getId() application ID}. If different applications need to access the same
     * pool of cached data, the same prefix should be set for each of the applications explicitly.
     */
    public $keyPrefix;
    /**
     * @var boolean whether to md5-hash the cache key for normalization purposes. Defaults to true. Setting this property to false makes sure the cache
     * key will not be tampered when calling the relevant methods {@link get()}, {@link set()}, {@link add()} and {@link delete()}. This is useful if a Yii
     * application as well as an external application need to access the same cache pool (also see description of {@link keyPrefix} regarding this use case).
     * However, without normalization you should make sure the affected cache backend does support the structure (charset, length, etc.) of all the provided
     * cache keys, otherwise there might be unexpected behavior.
     * @since 1.1.11
     **/
    public $hashKey = true;
    /**
     * @var array|boolean the functions used to serialize and unserialize cached data. Defaults to null, meaning
     * using the default PHP `serialize()` and `unserialize()` functions. If you want to use some more efficient
     * serializer (e.g. {@link http://pecl.php.net/package/igbinary igbinary}), you may configure this property with
     * a two-element array. The first element specifies the serialization function, and the second the deserialization
     * function. If this property is set false, data will be directly sent to and retrieved from the underlying
     * cache component without any serialization or deserialization. You should not turn off serialization if
     * you are using {@link CCacheDependency cache dependency}, because it relies on data serialization.
     */
    public $serializer;

    /**
     * Initializes the application component.
     * This method overrides the parent implementation by setting default cache key prefix.
     */
    public function init()
    {
        parent::init();
        if ($this->keyPrefix === null)
            $this->keyPrefix = Yii::app()->getId();
    }

    /**
     * @param string $key a key identifying a value to be cached
     * @return string a key generated from the provided key which ensures the uniqueness across applications
     */
    protected function generateUniqueKey($key)
    {
        return $this->hashKey ? md5($this->keyPrefix . $key) : $this->keyPrefix . $key;
    }

    /**
     * Retrieves a value from cache with a specified key.
     * @param string $id a key identifying the cached value
     * @return mixed the value stored in cache, false if the value is not in the cache, expired or the dependency has changed.
     */
    public function get($id)
    {
        $value = $this->getValue($this->generateUniqueKey($id));
        if ($value === false || $this->serializer === false)
            return $value;
        if ($this->serializer === null)
            $value = unserialize($value);
        else
            $value = call_user_func($this->serializer[1], $value);
        if (is_array($value) && (!$value[1] instanceof ICacheDependency || !$value[1]->getHasChanged())) {
            Yii::trace('Serving "' . $id . '" from cache', 'system.caching.' . get_class($this));
            return $value[0];
        } else
            return false;
    }

    /**
     * Retrieves multiple values from cache with the specified keys.
     * Some caches (such as memcache, apc) allow retrieving multiple cached values at one time,
     * which may improve the performance since it reduces the communication cost.
     * In case a cache does not support this feature natively, it will be simulated by this method.
     * @param array $ids list of keys identifying the cached values
     * @return array list of cached values corresponding to the specified keys. The array
     * is returned in terms of (key,value) pairs.
     * If a value is not cached or expired, the corresponding array value will be false.
     */
    public function mget($ids)
    {
        $uids = array();
        foreach ($ids as $id)
            $uids[$id] = $this->generateUniqueKey($id);

        $values = $this->getValues($uids);
        $results = array();
        if ($this->serializer === false) {
            foreach ($uids as $id => $uid)
                $results[$id] = isset($values[$uid]) ? $values[$uid] : false;
        } else {
            foreach ($uids as $id => $uid) {
                $results[$id] = false;
                if (isset($values[$uid])) {
                    $value = $this->serializer === null ? unserialize($values[$uid]) : call_user_func($this->serializer[1], $values[$uid]);
                    if (is_array($value) && (!$value[1] instanceof ICacheDependency || !$value[1]->getHasChanged())) {
                        Yii::trace('Serving "' . $id . '" from cache', 'system.caching.' . get_class($this));
                        $results[$id] = $value[0];
                    }
                }
            }
        }
        return $results;
    }

    /**
     * Stores a value identified by a key into cache.
     * If the cache already contains such a key, the existing value and
     * expiration time will be replaced with the new ones.
     *
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @param ICacheDependency $dependency dependency of the cached item. If the dependency changes, the item is labeled invalid.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     */
    public function set($id, $value, $expire = 0, $dependency = null)
    {
        Yii::trace('Saving "' . $id . '" to cache', 'system.caching.' . get_class($this));

        if ($dependency !== null && $this->serializer !== false)
            $dependency->evaluateDependency();

        if ($this->serializer === null)
            $value = serialize(array($value, $dependency));
        elseif ($this->serializer !== false)
            $value = call_user_func($this->serializer[0], array($value, $dependency));

        return $this->setValue($this->generateUniqueKey($id), $value, $expire);
    }

    /**
     * Stores a value identified by a key into cache if the cache does not contain this key.
     * Nothing will be done if the cache already contains the key.
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @param ICacheDependency $dependency dependency of the cached item. If the dependency changes, the item is labeled invalid.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     */
    public function add($id, $value, $expire = 0, $dependency = null)
    {
        Yii::trace('Adding "' . $id . '" to cache', 'system.caching.' . get_class($this));

        if ($dependency !== null && $this->serializer !== false)
            $dependency->evaluateDependency();

        if ($this->serializer === null)
            $value = serialize(array($value, $dependency));
        elseif ($this->serializer !== false)
            $value = call_user_func($this->serializer[0], array($value, $dependency));

        return $this->addValue($this->generateUniqueKey($id), $value, $expire);
    }

    /**
     * Deletes a value with the specified key from cache
     * @param string $id the key of the value to be deleted
     * @return boolean if no error happens during deletion
     */
    public function delete($id)
    {
        Yii::trace('Deleting "' . $id . '" from cache', 'system.caching.' . get_class($this));
        return $this->deleteValue($this->generateUniqueKey($id));
    }

    /**
     * Deletes all values from cache.
     * Be careful of performing this operation if the cache is shared by multiple applications.
     * @return boolean whether the flush operation was successful.
     */
    public function flush()
    {
        Yii::trace('Flushing cache', 'system.caching.' . get_class($this));
        return $this->flushValues();
    }

    /**
     * Retrieves a value from cache with a specified key.
     * This method should be implemented by child classes to retrieve the data
     * from specific cache storage. The uniqueness and dependency are handled
     * in {@link get()} already. So only the implementation of data retrieval
     * is needed.
     * @param string $key a unique key identifying the cached value
     * @return string|boolean the value stored in cache, false if the value is not in the cache or expired.
     * @throws CException if this method is not overridden by child classes
     */
    protected function getValue($key)
    {
        throw new CException(Yii::t('yii', '{className} does not support get() functionality.',
            array('{className}' => get_class($this))));
    }

    /**
     * Retrieves multiple values from cache with the specified keys.
     * The default implementation simply calls {@link getValue} multiple
     * times to retrieve the cached values one by one.
     * If the underlying cache storage supports multiget, this method should
     * be overridden to exploit that feature.
     * @param array $keys a list of keys identifying the cached values
     * @return array a list of cached values indexed by the keys
     */
    protected function getValues($keys)
    {
        $results = array();
        foreach ($keys as $key)
            $results[$key] = $this->getValue($key);
        return $results;
    }

    /**
     * Stores a value identified by a key in cache.
     * This method should be implemented by child classes to store the data
     * in specific cache storage. The uniqueness and dependency are handled
     * in {@link set()} already. So only the implementation of data storage
     * is needed.
     *
     * @param string $key the key identifying the value to be cached
     * @param string $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     * @throws CException if this method is not overridden by child classes
     */
    protected function setValue($key, $value, $expire)
    {
        throw new CException(Yii::t('yii', '{className} does not support set() functionality.',
            array('{className}' => get_class($this))));
    }

    /**
     * Stores a value identified by a key into cache if the cache does not contain this key.
     * This method should be implemented by child classes to store the data
     * in specific cache storage. The uniqueness and dependency are handled
     * in {@link add()} already. So only the implementation of data storage
     * is needed.
     *
     * @param string $key the key identifying the value to be cached
     * @param string $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     * @throws CException if this method is not overridden by child classes
     */
    protected function addValue($key, $value, $expire)
    {
        throw new CException(Yii::t('yii', '{className} does not support add() functionality.',
            array('{className}' => get_class($this))));
    }

    /**
     * Deletes a value with the specified key from cache
     * This method should be implemented by child classes to delete the data from actual cache storage.
     * @param string $key the key of the value to be deleted
     * @return boolean if no error happens during deletion
     * @throws CException if this method is not overridden by child classes
     */
    protected function deleteValue($key)
    {
        throw new CException(Yii::t('yii', '{className} does not support delete() functionality.',
            array('{className}' => get_class($this))));
    }

    /**
     * Deletes all values from cache.
     * Child classes may implement this method to realize the flush operation.
     * @return boolean whether the flush operation was successful.
     * @throws CException if this method is not overridden by child classes
     * @since 1.1.5
     */
    protected function flushValues()
    {
        throw new CException(Yii::t('yii', '{className} does not support flushValues() functionality.',
            array('{className}' => get_class($this))));
    }

    /**
     * Returns whether there is a cache entry with a specified key.
     * This method is required by the interface ArrayAccess.
     * @param string $id a key identifying the cached value
     * @return boolean
     */
    public function offsetExists($id)
    {
        return $this->get($id) !== false;
    }

    /**
     * Retrieves the value from cache with a specified key.
     * This method is required by the interface ArrayAccess.
     * @param string $id a key identifying the cached value
     * @return mixed the value stored in cache, false if the value is not in the cache or expired.
     */
    public function offsetGet($id)
    {
        return $this->get($id);
    }

    /**
     * Stores the value identified by a key into cache.
     * If the cache already contains such a key, the existing value will be
     * replaced with the new ones. To add expiration and dependencies, use the set() method.
     * This method is required by the interface ArrayAccess.
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     */
    public function offsetSet($id, $value)
    {
        $this->set($id, $value);
    }

    /**
     * Deletes the value with the specified key from cache
     * This method is required by the interface ArrayAccess.
     * @param string $id the key of the value to be deleted
     * @return boolean if no error happens during deletion
     */
    public function offsetUnset($id)
    {
        $this->delete($id);
    }
}